Electrostatic discharge (ESD) refers to the phenomenon whereby an electrical current of high amplitude and short duration is discharged at the package nodes of an integrated circuit due to static charge build-up on the integrated circuit (IC) package or on a nearby object, such as a human being or an IC handling machine. Without ESD protection circuitry, an ESD event can damage the IC. Accordingly, circuit designers have developed ESD protection circuitry to discharge ESD currents in a short time in a nondestructive manner.
A diode string represents one type of ESD circuit that can be used to discharge ESD currents. The diode string is formed in bulk material of a semiconductor substrate by series-connecting P-N junctions typically formed in n-well regions. In particular, each n-well formed in the P-type bulk material is tapped via an n+ diffusion and is connected to the p+ terminal of the next diode. The combination of a P+ diffusion contained in an n-well over a P-type substrate forms a parasitic PNP transistor by default, such that the “diode string” is really a chain of PNP transistors. However, to dissipate a power event using such diode strings, the diode string is connected to each exposed terminal, and the size of the diode string is selected to dissipate worst-case ESD event, which results in the inclusion of multiple circuit components that are only used occasionally, if ever.